The present invention relates to a method for changing over to a different frequency at a cellular phone system, and a cellular phone system using the method, and a base station controlling apparatus in the system.
Description of the Related Art
At a cellular phone system using a CDMA (code division multiple access) system, in order to keep the channel capacity in the high level, its transmission power is controlled. Generally, the transmission power is controlled by using a value of SIR (signal to interference power ratio) showing in the following equation (1).SIR=S/(I+N)  (1)
In this, the S signifies desiring wave signal power, the I signifies interference power per band, and the N signifies noise power per band.
At the control of the transmission power, the reception side measures the reception SIR shown in the equation (1). When the measured result is smaller than a target SIR, the reception side requires the transmission side to increase the transmission power, and when the measured result is larger than the target SIR, the reception side requires the transmission side to decrease the transmission power. With this, its channel quality can be controlled in a certain value and the channel capacity can be kept in the high level.
Further, at the cellular phone system using the CDMA system, in case that plural carrier frequencies are allocated, a technology, which changes over its carrier frequency to a different carrier frequency, is important.
Generally, this technology for changing over to the different frequency is a technology to make the frequency utilization efficiency high, and has a purpose that a call drop phenomenon caused by the deterioration of the channel quality in the currently using carrier frequency is avoided.
First, the technology changing over to a different frequency is explained in the viewpoint of the system structure. In the explanation of prior arts, drawings showing in FIGS. 1 to 3, which are the drawings using at embodiments of the present invention, are used.
In FIG. 1, the structure of a cellular phone system, in which two carrier frequencies are used in one service area, is shown. In FIG. 2, the positions of the carrier frequencies in the upstream channel and the downstream channel are shown.
Base stations 10, 11, 20, and 21, mobile communication terminals 30, 31, and 32, and a base station controlling apparatus 60, which controls the base stations, are shown in FIG. 1.
In FIG. 1, the mobile communication terminal 30 can connect its channel to any of base stations 10, 11, 20, and 21. The carrier frequencies, which the mobile communication terminal 30 uses in the channel with the base stations 10 and 11, is a carrier frequency 101 at the upstream channel, and a carrier frequency 103 at the downstream channel shown in FIG. 2. And the carrier frequencies, which the mobile communication terminal 30 uses in the channel with the base stations 20 and 21, is a carrier frequency 102 at the upstream channel and a carrier frequency 104 at the downstream channel shown in FIG. 2.
And the judgement changing over to the different frequency, whether the mobile communication terminal 30 connects to the base stations 10 and 11 or the base stations 20 and 21 during its communication, is controlled by the base station controlling apparatus 60. And also the base station controlling apparatus 60 executes the control changing over to the different frequency.
In order to control the changing over to the different frequency, the base station controlling apparatus 60 has a function to receive traffic information in each of the carrier frequencies. In this, the control changing over to the different frequency by the base station controlling apparatus 60 is executed to all of the base stations and the mobile communication terminals, which are managed by the base station controlling apparatus 60, under designated conditions.
In FIG. 1, broadcast channels 40 and 41, downstream individual channels 50 and 51, and upstream individual channels 70 and 71 are also shown.
Next, two conventional control methods for changing over to a different frequency are explained.
First, a first conventional control method for changing over to a different frequency is explained. At the first conventional control method for changing over to the different frequency, the control changing over to the different frequency is executed corresponding to the reception quality at the broadcast channels 40 and 41.
In FIG. 1, it is assumed that the channel of the mobile communication terminal 30 has been connected to the base station 10 as the initial state. At this time, at the downstream channel of the mobile communication terminal 30, the currently using carrier frequency is 103, and the different carrier frequency to which the currently using carrier frequency is changed over is 104. And at the upstream channel, the currently using carrier frequency is 101, and the different carrier frequency to which the currently using carrier frequency is changed over is 102.
At this time, the mobile communication terminal 30 measures the reception quality Q_1 in the broadcast channel of the currently using carrier frequency 103 and the reception quality Q_2 in the broadcast channel of the different carrier frequency 104 to which the currently using carrier frequency is changed over, and compares the measured results. And at the time when the difference between the reception quality Q_1 and Q_2 satisfies the following inequality (2), the changing over to the different frequency is executed.Q_2−Q_1>Th—HO_Quality [dB]  (2)
The changing over to the different frequency signifies that the carrier frequency using currently is changed over to a different frequency. In this case, at the downstream channel, the carrier frequency is changed over from the carrier frequency 103 to the carrier frequency 104, and at the upstream channel, the carrier frequency is changed over from the carrier frequency 101 to the carrier frequency 102.
At the inequality shown in (2), the Th_HO_Quality signifies a judging threshold value at the control for changing over to a different frequency, and the value of the Th_HO_Quality is positive.
Generally, at the inequality (2), the smaller the value of the Th_HO_Quality is, the smaller the probability generating a call drop becomes. However, the load controlling the changing over to the different frequency is increased when the value of the Th_HO_Quality is set to be small. Therefore, an optimum value is set as the value of the Th_HO_Quality. By the description mentioned above, the first conventional control method for changing over to the different frequency was explained.
Next, a second conventional control method for changing over to a different frequency is explained. At the second conventional control method for changing over to the different frequency, the changing over to the different frequency is controlled corresponding to traffic.
First, in FIG. 1, it is assumed that the channel of the mobile communication terminal 30 has been connected to the base station 10 as the initial state. At this time, at the downstream channel of the mobile communication terminal 30, the currently using carrier frequency is 103, and the different carrier frequency to which the currently using carrier frequency is changed over is 104. And at the upstream channel, the currently using carrier frequency is 101, and the different carrier frequency to which the currently using carrier frequency is changed over is 102. And the base station controlling apparatus 60 measures traffic of the currently using carrier frequencies 101 and 103, and of the different carrier frequencies 102 and 104 to which the currently using carrier frequencies are changed over.
At this second conventional control method for changing over to the different frequency, the upstream channels are studied. When the traffic in the currently using carrier frequency 101 is increased, and the traffic becomes close to the channel capacity limit in the system, the reception quality of the mobile communication terminal whose channel has been connected by using the currently using carrier frequency 101 is deteriorated, and the probability generating a call drop becomes high.
Generally, when the traffic in the currently using carrier frequency exceeded a threshold value, the base station controlling apparatus 60 judges that the traffic in the currently using carrier frequency is high. And the base station controlling apparatus 60 restricts the carrier frequency being high traffic to not connecting new channels, for preventing the deterioration of the channel quality. Further, the base station controlling apparatus 60 makes the transmission rate low for the plural mobile communication terminals using the currently using carrier frequency and also makes the channel capacity giving to the other users low. In this, it is not desirable to make the transmission rate low, because the throughput in each of the mobile communication terminals is made to be low.
In case that the high traffic occurred in the currently using carrier frequency, when it is possible to use other carrier frequency, the currently using carrier frequency is changed over to a carrier frequency in which the traffic is not high by the control changing over to the different frequency. With this, the deterioration of the channel quality can be avoided.
Next, the control changing over to the different frequency corresponding to the traffic is explained.
The base station controlling apparatus 60 observes the traffic in the currently using carrier frequency 101. When the traffic in the currently using carrier frequency exceeded a judging threshold value Th_Load, the base station controlling apparatus 60 instructs the mobile communication terminal 30 to measure the reception quality Q_2 of the broadcast channel of a different carrier frequency to which the currently using carrier frequency is changed over. The mobile communication terminal 30, which received this instruction, measures the reception quality Q_2 of the broadcast channel of the different carrier frequency to which the currently using carrier frequency is changed over. And the mobile communication terminal 30 informs the base station controlling apparatus 60 about the measured result.
The base station controlling apparatus 60, which received the measured result, controls to change over from the currently using carrier frequency to the different carrier frequency, when the measured result of the reception quality Q_2 of the broadcast channel of the different carrier frequency to which the currently using carrier frequency is changed over at the mobile communication terminal 30 satisfied the following inequality (3).Q_2>Th_HO_Load  (3)
In this, in the inequality (3), the Th_HO_Load is a judging threshold value. In the inequality (3), the higher the judging threshold value Th_HO_Load becomes, the higher the reception quality Q_2 in the broadcast channel after the carrier frequency was changed over becomes. However, when the judging threshold value Th_HO_Load is made to be high, the probability satisfying the condition in the inequality (3) becomes low, and the probability lowering the transmission rate becomes high. Therefore, the judging threshold value Th_HO_Load is set to be a value that is slightly larger than a level with which the reception quality Q_2 of the broadcast channel becomes a state being possible to communicate. This setting of the value comes from the following reason. That is, the probability executing the control by the low transmission rate is made to be low, within the range that the reception quality Q_2 of the broadcast channel of the different carrier frequency to which the currently using carrier frequency is changed over is in the state being possible to communicate. In this, the measurement of the traffic by the base station controlling apparatus 60 can be executed for the carrier frequency in the downstream channel. By the description mentioned above, the second conventional control method for changing over to the different frequency was explained.
Generally, at the cellular phone system using plural carrier frequencies, the mobile communication terminal in the system uses one oscillator from the viewpoint of low power consumption, and the oscillating frequency of the oscillator is changed, when it is needed. Under this structure of the mobile communication terminal, when the reception quality of the different carrier frequency, to which the currently using carrier frequency is changed over, is measured, there is a following problem due to the one oscillator. That is, the mobile communication terminal cannot receive data from the currently using carrier frequency while the mobile communication terminal is measuring the reception quality of the different carrier frequency to which the currently using carrier frequency is changed over.
Therefore, at the system, it is necessary that its transmitting waveform has been formed, at the state that a data vacant time for measuring the reception quality of the different carrier frequency to which the currently using carrier frequency is changed over is kept beforehand.
In case that the communication speed is a constant, in order to make the data vacant time, a data compression technology, by which the data vacant time is made, is required.
Generally, at the data compression technology, the transmission data are compressed in the time by using a method lowering its diffusion rate or a method making its coding rate higher by that a part of coded data is not transmitted. At the data compression technology, when it is compared with a case in which the data compression technology is not used, the error correction ability at the reception is deteriorated, therefore the frequency utilization efficiency is lowered. Consequently, it is not desirable that the ratio of the data vacant time to the communication time becomes high. Hereinafter, the ratio of the data vacant time to the communication time is referred to as the ratio of the data vacant time.
By the reason mentioned above, at the time of measuring the different frequency, it is desirable that the timing of changing over to the different frequency is judged accurately in a low ratio of the data vacant time. In order to lower the ratio of the data vacant time, the following improved control method is used for the first conventional control method for changing over to the different frequency mentioned above.
Next, this improved method, in which the range of the measurement of the different frequency is limited, is explained. In FIG. 3, at the downstream channel to the mobile communication terminal 30, the waveform of the reception quality Q_1 of the broadcast channel in the currently using carrier frequency and the waveform of the reception quality Q_2 of the broadcast channel in the different carrier frequency to which the currently using carrier frequency is changed over are shown.
In FIG. 3, in order to decrease the data vacant time at the time when the different frequency (reception quality Q_2) is measured, the mobile communication terminal 30 starts to measure the different frequency (reception quality Q_2) at the time t1 or t3, when the reception quality Q_1 of the broadcast channel in the currently using carrier frequency satisfied the following inequality (4).Q_1<Th_Start  (4)
After this, the mobile communication terminal 30 ends the measurement of the different frequency at the time t2, when the reception quality Q_1 of the broadcast channel in the currently using carrier frequency satisfied the following inequality (5). Or the mobile communication terminal 30 ends the measurement of the different frequency at the time t4, when the inequality (2) was satisfied, at the same time the control of the changing over to the different frequency is started.Q_1>Th_End  (5)
In this case, the judging threshold values Th_Start and Th_End in the inequalities (4) and (5) are desirable to have a margin satisfying the following inequality (6).Th_End−Th_Start>0  (6)
In the inequality (6), the larger the difference between the Th_End and Th_Start is, the larger the margin becomes. Generally, when the margin becomes large, the number of times of start and end of the measurement of the different frequency is decreased, and there is an advantage that the controlling load at the base station controlling apparatus 60 is decreased. However, since the number of times of the end of the measurement of the different frequency is decreased, the ratio of the data vacant time is increased, and the channel capacity is decreased. Therefore, the margin at the inequality (6) must be an optimum value.
By the concept mentioned above, the range of measuring the different frequency is limited by using the judging threshold values Th_Start and Th_End. With this, when it is compared with that the range of measuring the different frequency is not limited, the ratio of the data vacant time can be decreased.
As mentioned above, at the conventional methods for changing over to the different frequency, all of the base stations and all of the mobile communication terminals, which are managed by the base station controlling apparatus, are controlled under designated conditions. That is, each of the judging threshold values is a constant. Therefore, regardless of the high or low traffic in each of the carrier frequencies, the same control of changing over to the different frequency is applied. Consequently, when the traffic in a carrier frequency becomes high, the high traffic in the carrier frequency becomes close to the maximum limit of the channel capacity, and the probability generating a call drop becomes high. However, the probability changing over to the different frequency does not increase.
On the other hand, when the traffic in the carrier frequency becomes low, the margin in the channel capacity becomes large, and the probability generating the call drop becomes low. However, since the probability changing over to the different frequency does not decrease, the load for the base station controlling apparatus does not decrease, and the throughput at the base station controlling apparatus becomes low.
Moreover, at the conventional methods for changing over to the different frequency, all of the base stations and all of the mobile communication terminals, which are managed by the base station controlling apparatus, are controlled under designated conditions. That is, each of the judging threshold values is a constant. Therefore, regardless of that the transmission rate in each of the mobile communication terminals is high or low, the same control of changing over to the different frequency is applied. Consequently, when the transmission rate in each of the mobile communication terminals becomes high, its using capacity becomes close to the maximum limit of the channel capacity, and the probability generating the call drop becomes high. However, the probability changing over to the different frequency does not increase.
And when the transmission rate in each of the mobile communication terminals becomes low, the margin in the channel capacity becomes large, and the probability generating the call drop becomes low. However, since the probability changing over to the different frequency does not decrease, the load for the base station controlling apparatus does not decrease, and the throughput at the base station controlling apparatus becomes low.
Consequently, at the conventional technologies, there is a problem that it is difficult to utilize the processing ability of the base station controlling apparatus effectively with lowering the probability generating the call drop.